This invention relates to continuous dynode electron multipliers ("CDEMs"). More particularly, it deals with increasing the surface area within the channel in order to reduce the number of bombardments per unit area; this ultimately results in increased device lifetime.
Continuous dynode electron multipliers have been used for years to amplify electron, ion, or photon signals. When the inside diameter is 1 mm, typically electron multiplication (gain) of 10.sup.8 is possible.
A particle of sufficient energy can be detected when it is incident upon the inner surface of a CDEM channel and causes the emission of at least one secondary electron. As this secondary electron is accelerated "downstream" by the electrostatic field within the channel, it gains energy. When the electron gains sufficient energy, it will release one or two secondary electrons when it strikes the inner surface again. This process may occur up to 10 to 20 times in a CDEM, depending on such factors as length of the multiplier and applied voltage. Since the number of electrons is continuously increasing, this number of particles (and consequently the number of bombardments) reaches its maximum at the output of the CDEM.
The channel of the CDEM is made of a secondary-emissive surface so designed to increase the chances that an electron will be given off. Typically, this secondary-emissive surface is a thin coating on the inner glass channel. Since it is just a thin coating, when it is struck by these particles, there is a gradual degradation of the secondary-emissive surface. Thus, this coating has a limited life; and after too many bombardments, it loses its high secondary-electron yield and renders the entire channel useless.
As a result of the increased number of electrons near the end of the channel, the number of bombardments against the inner channel likewise increases. Consequently, the surface of the rear portion of the channel takes excessive abuse; and degradation of this surface area becomes a key manufacturing and design problem. Although the typical lifespan of a multiplier is one year for normal operation, it may not last this long if it is driven excessively.
Because of this excessive wear-and-tear on the output end of the channel, a replaceable-end CDEM was invented; see pending U.S. patent application Ser. No. 07/320277, filed Mar. 6, 1989, by all of the present Applicants and two additional co-inventors. As mentioned in that application, Applicants first recognized that there are occasions when the rear portion of the channel has become useless due to excessive bombardments, while the input end remains quite usable. So, they (and their two co-inventors) invented a channel that had a replaceable end section which could be discarded when it was sufficiently worn. That invention ultimately resulted in a commercially available electron multiplier sold by Detector Technology, Inc., of Brookfield, Mass. With it there is less equipment down-time and, more importantly, cheaper costs to the user since only the defective part has to be replaced, not the entire multiplier.
This application is an extension of Applicants' earlier recognition of what causes a short CDEM lifetime --namely, excessive degradation in the channel caused by heavy electron bombardment. Applicants have now created another means to solve that problem and increase the life span of the multiplier.
Even though the design is quite different than present-day CDEMs, this new CDEM retains the same high gain typical of present-day CDEMs. Likewise, other characteristics of CDEMs, such as the signal-to-noise ratio, are virtually identical to those CDEMs presently in common use.
Applicants' research discloses that the electron gain of a CDEM diminishes in close relation to the electron charge which passes through the channel. Therefore, Applicants believe a new and unique method for extending the device lifetime is to increase the active surface area as the charge advances from the input to the output of the CDEM. By increasing the inner surface area over that of a typical multiplier (while maintaining approximately the same overall detector dimensions), the number of bombardments per unit area decreases (assuming a constant input signal). Consequently, the damage on the inner channel is less per area unit, which results in longer lifetime for the device.
There have been several prior electron multipliers with different shaped channels in the past; however, none have dealt specifically with increasing the surface area in order to reduce the number of partical bombardments per unit area.
Because the total damage to the surface is proportional to the integrated number of electron bombardments per unit area, it is a primary object of this invention to increase the surface area within the channel, therefore increasing the lifetime of the device.
It is another object of this invention to provide for an increased output current (signal) capability (dynamic range) as a result of an increased surface area.
It is yet another object to provide an improved electron multiplier which retains the high quality of common CDEMs and is likewise durable and reliable.